M4220:
Thanks for sharing the wonderful Germanic double. How I wish I had one like this.
Can't wait to read how it shoots.
For the Rook, try cast bullets of about 85 to 100 grains designed for the 30 mauser. Some are available in .308. Nominally the Rook was to shoot a .300 bullet, so these could be sized a few thousandths or until the seated bullet and case fits when chambered. I have used Lee 7.62 nagant revolver dies to reload my .310 Cadet. This should work for the Rook if dies are an issue.
Have you cast the chambers and done any measuring of the 500 Bore? What a great hunter this one will be. Elegant, fast and powerful.